"Open door" lesson observation policies encouraged by many schools are causing disruption to classes and are detrimental to learning, the Association of Teachers and Lecturers (ATL) conference heard this week.
Delegates voted in favour of a motion calling for an investigation of how "drop-ins" are being used by schools, to ensure they are not another way of checking up on staff.
Along with formal lesson observations by Ofsted and senior management, teachers are increasingly having to welcome colleagues on "learning walks" designed to gauge cross-school issues, while some schools are encouraging increased use of "peer observation" by teacher colleagues.
Teacher Liz Smith attacked the Government for pressing ahead with plans to lift the three-hour annual time limit on lesson observation of an individual teacher by headteachers.
Ms Smith, from York High School, told the conference there was now a proliferation of reasons why teachers' lessons could be interrupted - often at key times such as during work on key stage 3 projects.
From students dropping in to find out about teaching to assistant heads making uniform checks, she said, chunks of lessons were now being devoured by these visits.
She also claimed some teachers were receiving six visits a week. "I don't have a problem in general with other people being in my classroom," she told the conference. "I also see some benefits to many of the schemes being put forward.